In contrast to the "wireless" intercom system of the present invention, door answering intercom systems currently in use that do not require the visitor to manipulate a push-to-talk button at the outside door station are all of the "wired" type. In other words, special wires must be run between each inside answering station and the outside door station to carry the necessary audio electrical signals back and forth between the outside and the inside stations.
Such a system, with its special wiring, can be easily installed in a building that is under construction. However, it is much more difficult to install the necessary wiring of this type running from the outside door calling station to the inside door answering station (or stations) in existing buildings. This wiring problem is especially troublesome for the "do-it-yourselfer."
"Wireless" intercommunication systems that utilize the basic electrical wiring of a building to transmit a radio frequency signal within the building--producing what is called "carrier current transmission" --are very well known. Wireless intercom systems of the AM or amplitude modulation type have been available for a great many years. See, for example, the U.S. Pat. Nos. to Benson 1,840,013 issued in 1932, Parker 2,221,994 issued in 1940, Roy 2,883,459 issued in 1959 and Liberman 2,887,533 also issued in 1959. Wireless intercom systems of the FM or frequency modulation type have also been known for a number of years. See, for example, U.S. Pat. No. 4,701,945 issued to Pedigo in 1987 on an application filed in 1984.
However, all wireless intercom models presently known are either (1) of the "VOX" (voice activated switching) type or (2) of the type that requires the person at each station to push the "talk" button to talk, then release it to listen. With the first type, the user's voice puts the radio unit in its transmitting condition. With the second type, pushing the "talk" button switches the radio unit from its receiving to its transmitting condition, and releasing the button does the reverse.
The "VOX" systems are not suitable for door answering intercoms since other sounds beside voice, such as street traffic noises, will switch the "VOX" units to "send." Furthermore, if both parties speak at once both stations will switch to the "send" or "transmit" mode, and then neither party will hear the other.
The pushing of buttons is acceptable for office intercoms or even for room-to-room household intercoms, where all users can be made to know and understand the "push-to-talk/release-to-listen" system. However, it is not practical for door answering systems, since a visitor to a home or other building who is standing at the outside door does not know that he must push the "push-to-talk" button after he rings the door bell. Signs and plaques at the door with instructions, or verbal instructions over the intercom, are cumbersome, and make known types of wireless intercom systems unsalable for door answering use.